1. Field of the Invention
The present invention relates generally to a process for manufacturing a grooved substrate and to a multilayer structure and more particularly to a grooved substrate and multilayer structure especially suitable for optical disks.
2. Description of the Prior Art
Microstructuring of thin dielectric layers or processes for manufacturing grooved substrates are needed in the whole data processing technology. Applications are to be found not only in the semiconductor and packaging technology but also in integrated optics technology, in the storage technology, e.g. for data communications, for magnetic disks and especially for optical disks. The high storage density of optical disks is achieved by using beam diameters of approximately 1 .mu.m. The information is recorded at a predetermined position and read out from a predetermined position Therefore the beam position needs to be accurately controlled by the use of guide addresses or guide signals. These typically are microscopic grooves, which in conjunction with a sensing mechanism and a servo system operating upon the optical beam serve to guide the beam in the correct direction during recording and reading operations. Various methods for forming the grooves on the optical disk have been proposed.
PCT/EP88/00479 describes a hot stamping process for forming the tracking grooves directly in the glass surface of an optical disk. According to this method the glass substrate is heated in excess of the softening point of the glass types used, typically higher than 600.degree. C. The temperature of the heated stamp should preferably be lower than the transformation temperature of the glass type used, typically 380.degree.-450.degree. C. The microstructure on the heated stamper deforms the surface of the glass substrate resulting in a corresponding microstructure on the surface of the glass substrate, which, when separated from the stamper and cooled in a controlled manner, gives the desired surface microstructure on the glass substrate. Microstructures formed on the surface of the glass substrate using the hot stamping process as described above conform to the microstructure on the stamper across the area of the substrate surface, but, for example, the glass may flow into a deep groove in the stamper to give a shallow protrusion on the substrate surface. Since for reasons of economy and product cost reduction it is desired to re-use the stamper a number of times the hot stamping process needs the selection of appropriate stamping conditions and coating materials for the glass layer to enable a single stamper to be used repeatedly without sticking problems. The high temperatures applied to the glass substrate during hot stamping processes, e.g. higher than 600.degree. C., may cause the deformation of the glass substrate.
The method for manufacturing substrates having a large number of fine grooves thereon as described in U.S. Pat. No. 4,810,547 comprises applying a solution containing at least one organometal compound and a thickening agent onto a body of substrate to form a film having plasticity, impressing in the surface of the film formed on the substrate with a mold and calcining the film to solidify the same. A similar method is claimed in the unexamined Japanese patent application JP 62-102445.
These methods provide advantages in that the resulting film has a shrinkage lower than that of gel films as for example used in PCT/GB88/01080 and seldom causes cracks and/or warpage due to nonuniform drying rate since the organometal compound is formed in a film on the substrate body.
When used for optical disks grooved substrates and structures should have a high refractive index and a high signal to noise ratio in addition to the already mentioned properties. This normally is achieved by producing a multilayer structure on a grooved substrate, as described by Takahashi, et al. in "High Quality Magneto-Optical Disk", SPIE Vol. 695 Optical Mass Data Storage II (1986).
There has been a strong need for substrates with fine grooves, especially for optical disks, which do not show the various disadvantages aforementioned and which, on the contrary, have an extremely high dimensional stability and reliability as well as a high productivity in their manufacturing process.